The present invention refers to an apparatus for fluidizing and transferring powdered or pulverulent material, and in particular to an apparatus for fluidizing powdered material contained in a container and transporting the fluidized powder directly to a spray gun of an electrostatic powder coating plant.
German utility model G 88 04 951 describes an apparatus of this type including a suction and transport pipe which is introduced into the container through its open top, and a compressed air pipe which is also introduced from above into the container and is provided with air openings for generating a locally limited powdery fluid bed. Such apparatus allows the conveyance and supply of powder directly from the original container to the spray gun without requiring any intermediate receptacle. Practice has shown however that this apparatus operates satisfactorily only when the powder is in perfect conditions. i.e. free of lumps, free of foreign particles and free of oversized powder particles which considerably exceed the predetermined average grain size. However, such conditions do no always prevail. For example, through a lengthy storage of the powder within the original container or temperature fluctuations formation of powder lumps may be encountered or particles of excessive grain size may be contained. The presence of such powder lumps or oversized powder particles results in an uneven coating of the workpiece which diminishes the quality especially when considering a finish coating. Practice has further shown that formation of undesired lumps is even more prevalent when employing the apparatus of the type disclosed in the German utility model G 88 04 951 for withdrawing and transferring powder which has been sprayed already i.e. recycled powder.
It was therefore suggested to interpose a filter of suitable fineness within the suction and transport pipe before the powder reaches the spray gun, e.g. at the suction port or within any other suitable location of the pipe. This proposal has proven fruitless because such a filter becomes clogged already after a short period. Further, it should be taken into account that, in general, an apparatus of this type operates at very low suction force so that an increased suction effect for freeing a filter from cloggings would considerably interfere with the withdrawal of powder from the fluid bed. Also, the installation of an intermediate chamber which contains a screening machine in the suction and transport pipe has proven to be too complicated as the powder which passes the screen of the screening machine has to be fluidized and transported again. The previously stated advantage of direct withdrawal and transport of powder from the original container to the spray gun and thus the advantage of reduced constructive design would be eliminated.